1. Field of the Invention
The present invention relates to a piston-cylinder unit installed between a vehicle body and a wheel guide part.
2. Description of the Related Art
A prior art piston-cylinder unit, which is preferably embodied as a vibration damper, is known from EP 0 501 115 A1. The prior art unit is part of a controllable suspension system for wheel suspensions of motor vehicles. The suspension system comprises a first spring and a hydraulic vibration damper that operates parallel to the first spring as well as a second spring that also operates parallel to the first spring. In one embodiment, the first spring surrounds the vibration damper over the entire installation length, while the second spring is connected to a hydraulic piston of a switchable hydraulic system. The switching valve automatically closes upon power cutoff, so that the second hard spring is activated, resulting in a hard suspension and thus in reliable driving performance. It is also known to change the spring pre-bias in vehicle suspensions and to arrange a non-adjustable stop buffer, which is connected to the cylinder and surrounds the piston rod, in vibration dampers.
Also known are hydropneumatic suspension and damping systems with hydropneumatic accumulators separated from the spring cylinder. These systems are usually designed with an inherent frequency of approximately 1 Hz in the different permissible load states. In addition, the accumulator volume and its gas pre-stress are such that the diaphragm separating the gas space from the fluid space is prevented from coming to rest on either the gas-side or the fluid-side inner wall in any operating state.